The present invention relates to a method to devide a beam being suited to be applied for an equipment by means of which a beam is devided in such a manner the one beam is made use of for example in order to form the image while the other beam is made use of for light measuring, and more particularly to a method to control introduction of the diffracted beam by means of diffraction element, making a beam enter into a diffraction grating structure and leading the diffracted beam along certain determined directions.
It has already been known to carry out the light measurement taking out a part of the image forming beam having informations to form an image whereby as the means to devide the beam a half-mirror or a perforated mirror has been used, while at times on the surface of a transparent solid body a small prism has been provided in such a manner that a part of beam be reffracted by means of the prism.
However it has been inevitable that the formed image should become considerably dark to install a perforated mirror in the image forming beam. Further in order to install a mirror obliquely in the image forming beam much space is needed, while the space is limited inside of an optical instrument for the installation of a mirror, which is disadvantageous.
On the other hand it is inevitable to produce a bad influence of the image forming ability to install a transparent solid body having a small prism in the image forming light flux.
Further in order that the light measurement might be carried out at the same time at several parts of the image forming beam or that the light measurement might be carried out at certain determined important parts of the beam, the manufacture of the light deviding means has been extremely difficult.